Variable thickness diaphragm for a wideband robust piezoelectric micromachined ultrasonic transducer (pmut)

ABSTRACT

A diaphragm for a piezoelectric micromachined ultrasonic transducer (PMUT) is presented having resonance frequency and bandwidth characteristics which are decoupled from one another into independent variables. Portions of at least the piezoelectric material layer and backside electrode layer are removed in a selected pattern to form structures, such as ribs, in the diaphragm which retains stiffness while reducing overall mass. The patterned structure can be formed by additive, or subtractive, fabrication processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §111(a) continuation of PCTinternational application number PCT/US2015/018076 filed on Feb. 27,2015, incorporated herein by reference in its entirety, which claimspriority to, and the benefit of, U.S. provisional patent applicationSer. No. 61/945,906 filed on Feb. 28, 2014, incorporated herein byreference in its entirety. Priority is claimed to each of the foregoingapplications.

The above-referenced PCT international application was published as PCTInternational Publication No. WO 2015/131083 on Sep. 3, 2015, whichpublication is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF COMPUTER PROGRAM APPENDIX

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND

1. Technological Field

This technical disclosure pertains generally to ultrasonic transducers,and more particularly to an ultrasonic transducer which can be readilyconfigured for a desired dynamic response.

2. Background Discussion

Piezoelectric micromachined ultrasonic transducers (PMUTs) aremicro-electro-mechanical system (MEMS) devices which operate in responseto flexural motion of a thin membrane coupled with a thin piezoelectricfilm, instead of thickness-mode motion of a plate of piezoelectricceramic as within bulk piezoelectric ultrasound transducers. It will benoted that PMUTs are a class of micromachined ultrasonic transducers(MUTs). In comparison with bulk piezoelectric ultrasound transducers,PMUTs can offer advantages such as increased bandwidth, flexiblegeometries, natural acoustic impedance match with water or air, reducedvoltage requirements, mixing of different resonant frequencies andpotential for integration with supporting electronic circuits especiallyfor miniaturized high frequency applications.

However, despite the intensive study of MUTs in recent decades, mostPMUT designs adhere to clamped square and circular plates, or if theyhave different mechanical configurations it is toward obtainingdifferent goals/objectives than in this disclosure. For instance,certain PMUT configurations are targeted to achieve piston-likemovement, in order to increase the output pressure and the active areaof the device. Yet, piezoelectric actuated transducers require acurvature mode shape in order to couple the electrical and themechanical energy efficiently. For example, a prior study performed bythe inventors on a circular flexurally-suspended PMUT had a piston-likemode shape. It was found that although the linear operating range wasincreased, the output pressure and bandwidth were compromised.

Therefore, a need exists for enhanced PMUT designs which provideincreased resonant frequency control, bandwidth and other enhancedoperating characteristics.

BRIEF SUMMARY

Piezoelectric micromachined ultrasonic transducers (PMUTs) are describedhaving a transducer surface which is patterned, so that both mass andstiffness of the diaphragm can be modified independently of one another.The technology presented is applicable to PMUTs, as well as MUTs ingeneral, and to other vibrating plate structures. This method allowstargeting the mechanical dynamic response of the transducer to therequired resonance frequency and bandwidth. This technique isimplemented for designing wideband piezoelectric micromachinedultrasonic transducers that have low sensitivity to residual stress inorder to enable the fabrication on a single chip or wafer of multiplePMUTs having the same center frequency, but can also be used to meetother dynamic requirements.

Further aspects of the presented technology will be brought out in thefollowing portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the technology without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The disclosed technology will be more fully understood by reference tothe following drawings which are for illustrative purposes only:

FIG. 1A and FIG. 1B are a pictorial cross-section and material layercross-section, respectively, of a conventional PMUT having apiezoelectric layer shown with a top electrode.

FIG. 2A and FIG. 2B are a pictorial cross-section and material layercross-section, respectively, of a patterned PMUT according to anembodiment of the present disclosure.

FIG. 3 is a pictorial cross-section of a square patterned PMUT accordingto an embodiment of the present disclosure.

FIG. 4 is a pictorial cross-section of a hexagonal patterned PMUTaccording to an embodiment of the present disclosure.

FIG. 5 is a pictorial cross-section of an elongate linear patterned PMUTaccording to an embodiment of the present disclosure.

FIG. 6 is a pictorial cross-section of an array of patterned PMUTsaccording to an embodiment of the present disclosure.

FIG. 7 is a plot of the resonance frequency's stress sensitivity versusthickness demonstrated according to an embodiment of the presentdisclosure.

FIG. 8 is a bar plot of resonance frequency sensitivity depending forvarious rib angles demonstrated according to an embodiment of thepresent disclosure.

FIG. 9 is a bar plot of quality factor in response to the use of variousrib angles demonstrated according to an embodiment of the presentdisclosure.

FIG. 10 is a plot of quality factor and sensitivity to residual stressfor a number of PMUT designs, including rib designs, thick flat plates,reference flat thin design, and flat plate analytic and 2D FEM designs.

FIG. 11 is a bar plot of sensitivity of resonance frequency to residualstress for various configurations demonstrated according to anembodiment of the present disclosure.

FIG. 12 is a bar plot of quality factor for various configurationsdemonstrated according to an embodiment of the present disclosure.

FIG. 13 is a bar plot of sound pressure level for various configurationsdemonstrated according to an embodiment of the present disclosure.

FIG. 14 is a bar plot of resonance frequency for various configurationsdemonstrated according to an embodiment of the present disclosure.

FIG. 15A through FIG. 15E are cross section views in steps forfabricating a patterned PMUT according to an embodiment of the presentdisclosure.

FIG. 16 is a plot of stress sensitivity with respect to fractionalbandwidth, showing independence of stiffness and mass for patternedPMUTs according to embodiments of the present disclosure.

FIG. 17A and FIG. 17B are top and magnified image views of a thin-ribPMUT according to an embodiment of the present disclosure.

FIG. 18A and FIG. 18B are bar plots of resonant frequency and fractionalbandwidth comparing flat PMUTs with ribbed PMUTs according to anembodiment of the present disclosure.

FIG. 19 is a bar plot of simulated and measured outputs comparing flatdiaphragms and ribbed diaphragms configured according to an embodimentof the present disclosure.

DETAILED DESCRIPTION

Dynamic response in a piezoelectric micromachined ultrasonic transducer(PMUT), namely its center frequency and bandwidth, is principallydetermined by its mechanical resonance. Therefore, dynamic response is afunction of planar geometry, boundary conditions, material propertiesand thickness. In a simple diaphragm with uniform thickness, theresonance frequency and its bandwidth are dependent variables.

In order to separate (decouple) these two dependent variables andtherefore enable design of transducers with a variety of dynamicresponses, the present disclosure patterns the diaphragm by selectivelyremoving or adding layers in pre-defined regions, which makes resonantfrequency and bandwidth substantially independent variables. Thisapproach also allows the stiffness of the diaphragm to be increased,thereby reducing the effect of stress on the center frequency of thediaphragm.

If the diaphragm mechanics are modeled as a lumped second ordermass-spring-damper (m, k, b) system, the center frequency ω and thebandwidth Δω can be written as:

ω=√{square root over (k/m)}  (1)

Δω=b/m   (2)

where the degree of freedom (DOF) is the center displacement of thediaphragm. A common metric to describe bandwidth is the quality factorQ=ω/Δω.

FIG. 1A through FIG. 1B illustrate an embodiment 10 of a referencedesign, denoted herein as ‘design 0’ having a support structure (handle)12, backside tube 14, flat constant-thickness diaphragm 16 configuredwith a top piezo surface 18. In the figure, one sees this piezo surfacecontinuing on both sides beneath a top electrode 20. The diaphragm 16 isshown in FIG. 1B to comprise a passive elastic layer 22, over which is ametal layer 24, upon which piezo layer 18 is disposed. The diaphragmthickness is uniform except for the material of deposited top electrode20.

The diaphragm has mass m₀, stiffness k₀ and damping b₀ which are afunction of the diaphragm area A₀ and thickness t₀:

m₀∝A₀t₀

b₀∝A₀

k₀∝t₀ ³/A₀   (3)

Therefore, the center frequency ω₀ and bandwidth Δω₀ are:

$\begin{matrix}{{\omega_{0} \propto \frac{t_{0}}{A_{0}}}{{\Delta \; \omega_{0}} \propto {\frac{1}{t_{0}}.}}} & (4)\end{matrix}$

Stress is known to result in variations in the effective stiffness ofthe diaphragm, which in turn results in changes in the center frequencyω₀ in much the same way that tension in a guitar string increases itsfrequency. A laminated diaphragm with a stressed AlN layer, will have acenter frequency ω_(0,res) that is shifted relative to that of anunstressed diaphragm ω₀:

$\begin{matrix}{\left( \frac{\omega_{0,{res}}}{\omega_{0}} \right)^{2} = \left\lbrack {1 + \frac{a^{2}\sigma \; t}{\lambda_{01}^{2}D}} \right\rbrack} & (5)\end{matrix}$

where a denotes the radius, σ the average stress, t the thickness, D ∝t³the flexural rigidity and λ₀₁ is a constant defined by the vibrationmode-shape of the diaphragm. This equation shows that, for a given levelof stress variation, it is possible to reduce the effect of stress oncenter frequency by increasing the thickness and therefore stiffness k₀,seen in Eq. 3. However, from Eq. 4, it will be noted that the centerfrequency can be kept constant when the thickness is increased byincreasing the area by the same proportion. Yet this approach has theundesirable effect of reducing the bandwidth Δω₀.

By removing mass from the diaphragm by selectively reducing itsthickness in various locations, the novel device structure describedhere allows the PMUT design to be targeted not only towards the requiredfrequency but also towards a specific bandwidth and stiffness. However,since mass reduction of the diaphragm also affects its stiffness, it isimportant to tailor the stiffness, area, and mass of the diaphragm tofit the required frequency, bandwidth, and stress sensitivity. Thisnovel configuration is referred to herein as ‘design 1’, so itsparameters mass m₁, stiffness k₁ and damping b₁ are functions of the newtotal diaphragm area A₁ and maximum thickness t₁:

$\begin{matrix}{{m_{1} \propto {\left( {1 - X} \right)A_{1}t_{1}}}{b_{1} \propto A_{1}}{k_{1} \propto {\frac{t_{1}^{3}}{A_{1}}\left( {1 - Y} \right)}}} & (6)\end{matrix}$

where X represents the mass reduction relative to the mass of a constantthickness diaphragm with the same planar geometry, and Y represents thestiffness reduction relative to that of a diaphragm with the same planargeometry.

Assuming that ‘design 1’ targets the same center frequency as ‘design0’, ω₁=ω₀, the bandwidth can be adjusted by the mass reduction andthickness as given by:

$\begin{matrix}{{\Delta \; \omega_{1}} = \frac{1}{t_{1}\left( {1 - X} \right)}} & (7)\end{matrix}$

while the size of the new design is determined by both the change instiffness and mass:

$\begin{matrix}{\frac{A_{0}}{t_{0}} = \frac{A_{1}\sqrt{\left( {1 - X} \right)}}{t_{1}\sqrt{\left( {1 - Y} \right)}}} & (8)\end{matrix}$

As demonstrated by Eqs. 6-8, introducing X and Y adds two new designvariables that broaden the design space.

FIG. 2A and FIG. 2B illustrate an example embodiment 30 of the disclosedtransducer (‘design 1’) having a thick diaphragm that is patterned onits surface with wedge-shaped ribs that reduce the mass while enhancingstiffness. The presented embodiment 30 has a support structure (handle)32, backside tube 34, a patterned thickness diaphragm 36 that isactuated using piezo layer 38 by applying voltage to top electrode 40.It should be appreciated that tube 34 is not required in the structure,only that some mechanism provide for holding the edges of the diaphragmfixed (e.g., wafer or support structure).

A series of “ribs” 39 are seen extending from a perimeter of the cutareas 48 toward a central cut area 46. In this example, each rib 39 isin a wedge shape (isosceles triangle) with its tip cut off, this shapebeing generally referred to as isosceles trapezoid. It should beappreciated, however, that other rib shapes may be utilized withoutdeparting from the present teachings. The preferred taper angles on thesides of the ribs may be anything from 0° (straight bar ribs) through to45°.

The diaphragm 36 is shown in FIG. 2B to comprise a passive (elastic)layer 42, over which is a conductive (metal) layer 44, upon which theactive piezo layer 38 is deposited. The top electrode 40 is a closedshape surrounding an open area, for example a closed-shape ring, alongthe non-patterned edges of the diaphragm, but can also be patterned onthe ribs to enhance the coupling of piezoelectric stress to diaphragmdisplacement, and other patterns are possible for top electrode 40. Thepatterning can be seen in the figure in which center region 46 alongwith interdigital areas 48, are substantially cut away, such as toremove the piezo and conductive layers, and in this specific exampleeven a major thickness portion of the passive layer is etched away. Thethickness of center region 46 is preferably in the range from 0.5microns to 4 microns, or more preferably between from 1 micron to 2microns. It should be appreciated that the patterning of the PMUT may befabricated using a subtractive material removal process (e.g., cutting,etching, ablation), or the pattern formed from an additive process, or acombination of subtractive and additive processes utilized to create thepatterned PMUT. The circular PMUT shape seen in FIG. 2A and FIG. 2B isshown by way of example, while the present disclosure may be implementedin a range of geometries without departing from the disclosure.

In the present disclosure the piezoelectric material is exemplified asAluminum Nitride (AlN), however, it will be appreciated that numerousmaterials exhibiting piezoelectric behavior may be alternativelyutilized without departing from the present teachings. By way of exampleand not limitation, material may be selected for use from the group ofmaterials exhibiting piezoelectric behavior comprising Apatite, BariumTitanate (BaTiO₃), Berlinite (AlPO₄), various Ceramic materials, AlliumPhosphate, Gallium Nitride (GaN), Gallium Orthophosphate, LanthanumGallium Silicate, Lead Scandium Tantalate, Lead Magnesium Niobate (PMN),Lead Zirconate Titanate (PZT), Lithium Tantalate, PolyvinylideneFluoride (PVDF), Potassium Sodium Tartrate, Quartz (SiO₂), Zinc Oxide(ZnO), and other materials and combinations as will be known to one ofordinary skill in the art. By way of example and not limitation, oneclass of ceramics materials exhibiting piezo electric properties areceramic structures exhibiting perovskite tungsten-bronze structures,including BaTiO₃, KNbO₃, Ba₂NaNb₅O₅, LiNbO₃, SrTiO₃, Pb(ZrTi)O₃,Pb₂KNb₅O₁₅, LiTaO₃, BiFeO₃, Na_(x)WO₃. Similarly, various materials maybe used for the elastic layer of the PMUT diaphragm, including forexample silicon, silicon nitride, and silicon dioxide. It will also beappreciated that an active piezoelectric material may be used for theelastic layer of the diaphragm. Numerous materials exhibitingpiezoelectric behavior may be alternatively utilized for the elasticlayer without departing from the present teachings, including AluminumNitride (AlN), Apatite, Barium Titanate (BaTiO₃), Berlinite (AlPO₄),various Ceramic materials, Allium Phosphate, Gallium Orthophosphate,Gallium Nitride (GaN), Lanthanum Gallium Silicate, Lead ScandiumTantalate, Lead Magnesium Niobate (PMN), Lead Zirconate Titanate (PZT),Lithium Tantalate, Polyvinylidene Fluoride (PVDF), Potassium SodiumTartrate, Quartz (SiO₂), Zinc Oxide (ZnO), and combinations thereof.

FIG. 3 through FIG. 5 illustrate additional example embodiments havingsquare, hexagonal and rectangular diaphragms.

In FIG. 3 a patterned square PMUT embodiment 50 is shown with handle 52,backside tube 54, patterned diaphragm 56 configured with a piezo layer58 beneath a square shaped top electrode 60. It can be seen thatmaterial is removed from the center 62, and in radial sections 64,leaving protruding rib sections 59, which are the only portionscontaining piezo layer 58 within the interior of the top electrode. Inthis example, one rib (e.g., isosceles trapezoid) is shown extendingfrom each of the sides of the square 60 perimeter.

In FIG. 4 a patterned hexagonal PMUT embodiment 70 is shown with handle72, backside tube 74, patterned diaphragm 76 configured with a piezolayer 78 beneath a hexagon-shaped top electrode 80. It can be seen thatmaterial is removed from the center 82, and in radial sections 84,leaving protruding rib sections 79, which are the only portionscontaining piezo layer 78 within the interior of the top electrode. Inthis example one rib (e.g., isosceles trapezoid) is shown extending fromeach of the sides of the hexagon 80 perimeter, although this is not alimitation of any of the embodiments. It should be appreciated that theabove PMUTs are examples of devices which are radially symmetric. Thepresent disclosure however, applies not only to any radially symmetricform, but to other forms/shapes as well.

In FIG. 5 is shown an example of other forms which are not radiallysymmetric. A patterned elongate linear PMUT embodiment 90 is shown withhandle 92, backside opening 94, patterned diaphragm 96 configured withpiezo layer 98. Structure 90 illustrates a top electrode 100 around itsperimeter. It can be seen that material is removed from interior regions102, leaving protruding rib sections 99, which are depicted here by wayof example as being rectangular shaped.

However, it should be appreciated that center frequency and bandwidthare not the only characteristics of an ultrasonic transducer. The rangeof a transducer is a function of the output pressure and its receiversensitivity, which should therefore be considered during the designprocess.

Implementations of the invention include piezoelectric micromachinedultrasonic transducers (PMUTs), or other vibrating diaphragm structures,for different applications, such as range and angle detection, flowmeasurements and medical imaging. Transducers of this type require ashort time constant that is equivalent to a wide bandwidth frequencyresponse. In addition, the medium in which the transducer is operating(e.g., air or other gases, fluids, solids), along with the maximum rangeover which the sound will propagate dictate operating at specificfrequencies. For example, air coupled transducers require centerfrequency between 40-800 kHz, while their bandwidth should be at least10% of the center frequency. Transducers can be designed with fractionalbandwidth Δω/ω between 1% and 40% or specifically between 5% and 25% ormore specifically between 5% and 15%. Transducers can be designed withcenter frequency from 20 kHz to 800 kHz or specifically from 40 kHz to450 kHz or more specifically from 80 kHz to 300 kHz.

FIG. 6 illustrates a PMUT array 110, with any desired number of PMUTs114 on the same substrate 112. By way of example and not limitation, thearray is shown with 25 PMUTs in a rectangular 5×5 pattern. It should beappreciated that PMUTs of any desired geometry may be utilized, whilethe array itself may comprise any desired PMUT arrangement (arraypattern), with any desired spacing. It should be appreciated that anarray of transducers may be desirable for generating sufficient pressureand/or for sensing applications, such as when measuring location andvelocity. In many applications, it is important for all the transducersin the array to vibrate at the same frequency, whereby that design mustaim for a very low variation in the center frequency due to residualstress or other fabrication tolerance. By increasing the total thicknessand removing mass from the center of the diaphragm, while adjusting thediaphragm dimensions, a PMUT can be designed according to the presentdisclosure which has a wide bandwidth and low sensitivity to residualstress.

It should be appreciated that the ribs may also be laser trimmed, in atleast one embodiment of the disclosure, to modify the mass and/orstiffness of the PMUT, thereby trimming the frequency. By way of exampleand not limitation, this fine tuning of rib dimensions may be performedto enhance frequency matching between PMUT devices in an array.

FIG. 7 depicts sensitivity and quality factor for an example flatdiaphragm PMUT design similar to the one shown in FIG. 1. Increasing thethickness, (moving to the right in the horizontal axis of the figure)reduces the sensitivity of the center frequency to stress but increasesquality factor and therefore reduces the bandwidth. The stresssensitivity of the center frequency and the quality factor were modeledusing finite element method (FEM) software.

Models of the new transducer design, as exemplified by FIG. 2A, werecreated for several different rib widths, using 3D, non-linear finiteelement method (FEM) eigenfrequency analyses.

FIG. 8 is a bar graph depicting how the sensitivity of the centerfrequency to stress was simulated for devices with 8 ribs having 5, 20,35, and 45 degree rib width. The flat design, marked ‘Flat’, has thesame 750 micron diameter and 5 micron diaphragm thickness as the totalthickness of the ribbed designs. The bar denoted Ref depicts aflat-plate reference design with 2 micron thickness and 400 microndiameter. Each design has a nominal center frequency of approximately200 kHz. The resonance frequency sensitivity was found to be 0.09kHz/MPa for the 5° rib width, 0.18 kHz/MPa for the 20° rib width, 0.23kHz/MPa for the 35° rib width, 0.23 kHz/MPa for the 45° rib width, 0.20kHz/MPa for flat transducer of same diameter and thickness, and 1.31kHz/MPa for the reference design with 2 micron thickness and 400 microndiameter. The center frequency of each thick-layered configuration isfrom 5 to 14 times less sensitive to residual stress than that of thethin reference design.

The bandwidth of the different configurations was found by studying thefrequency response of the transducer in air using a 3D piezo-acousticFEM model that couples the piezoelectric structure with the acousticmedium (air). The model comprises three-dimensional geometry of thePMUT, the structural piezoelectric materials, the air medium, and avoltage input at the electrode. The patterned diaphragm design describedhere results in a reduced-mass/enhanced-stiffness structure which allowsthe bandwidth to be increased, or equivalently the quality factor

$Q = \frac{\omega}{\Delta \; \omega}$

to be decreased.

FIG. 9 depicts predicted quality factor for the novel design withvarious angular rib widths (5°, 20°, 35°, 45°) compared to a flattransducer of the same dimensions and a reference design with 2 micronthickness and 400 micron diameter. The quality factors were found to be16.2 for the 5° rib width, 21.1 for the 20° rib width, 21.8 for the 35°rib width, 22.6 for the 45° rib width, 35.1 for flat transducer (samediameter and thickness), and 9.8 for the reference design with 2 micronthickness and 400 micron diameter. The quality factor found from the FEMmatches the experimentally measured quality factor of the referencedesign, validating the FEM model's ability to predict a given design'squality factor.

FIG. 10 depicts a plot of quality factor versus sensitivity of thecenter frequency to residual stress for the various designs. Ribbeddesigns are denoted by the open diamond shapes, with the thick flatdiaphragm 3D FEM seen by the filled diamond, a reference thin flatdiaphragm design is depicted by the circle in the lower right, with flatdiaphragm analytic analysis seen by the solid line, and the flatdiaphragm 2D FEM seen by the dotted line plot. It can be seen that insome cases, the ribbed designs have 1.5× to 3× lower quality factor andtherefore 1.5× to 3× greater bandwidth than flat designs havingequivalent stress sensitivity.

FIG. 11 depicts a bar graph of sensitivity of resonance frequency toresidual stress, shown for a non-removed piezo area PMUT (e.g., theprior art design shown in FIG. 1A and FIG. 1B), such as comprisingaluminum nitride (AlN), having 1.3 kHz/MPa, compared with a ringelectrode PMUT (having the piezo layer removed everywhere except beneatha ring electrode at the perimeter of the diaphragm) at 0.1 kHz/MPa, anda center electrode PMUT (having the piezo layer removed everywhere butbeneath a circular center electrode) having a sensitivity of 0.5kHz/MPa. It can be seen from the graph that using a patterned-diaphragmtransducer where the piezoelectric layer is removed from the surfaceexhibits smaller frequency variation with residual stress than theflat-diaphragm design. This patterned-diaphragm transducer has thepiezoelectric layer removed from the surface, except for either in thecenter of the diaphragm (referred to as a “Center Electrode” design) orin a ring surrounding the perimeter of the diaphragm (referred to as a“Ring Electrode” design).

FIG. 12 depicts a bar graph of quality factor comparing differenttransducer types. Quality factor is seen for the non-removed AlN at 10,a ring electrode at 17, and a center electrode at a quality factor of47. It is apparent that using a patterned-diaphragm transducer where thepiezoelectric layer is removed from the surface provides a higherquality factor.

FIG. 13 depicts output sound pressure level (SPL) (at 5 cm distance),normalized by the quality factor, in comparing the abovepatterned-diaphragm transducers. Sound pressure is seen as 48 dB/V forthe non-removed AlN, 43 dB/V for the ring electrode, and 44 dB/V for thecenter electrode. It is thus seen that the sound pressure is onlyslightly less for the ring electrode and center electrode types. Thelower pressure is due to reduced coupling of the piezoelectric straininto membrane displacement. Therefore, these configurations trade-offstress robustness for reduced bandwidth, unlike the thick ribs patternconfigurations.

FIG. 14 depicts the resonance frequency of each of these describedconfigurations, with resonance for the non-removed AlN at 228 kHz, thering electrode at 197 kHz, and the center electrode at 158 kHz. So thefundamental frequencies are seen to vary for a given size in each ofthese geometries. It will be appreciated, of course, that the size andthickness of each design type can be modified to achieve a desiredfundamental frequency.

FIG. 15A through FIG. 15E illustrate an example fabrication sequenceembodiment 130, in which fabrication is preferably performed utilizingstandard micromachining techniques. By way of example and notlimitation, one fabrication embodiment starts with a SOI(Silicon-On-Insulator) wafer having Si base layer 132 covered by adielectric layer (SiO₂) 134 as seen in FIG. 15A. Over the dielectriclayer 134 is a Si layer 136 that will serve as the elastic (passive)layer of the PMUT diaphragm. A bottom electrode layer (e.g., metal) isseen 138, over which is a piezo layer 140.

In the fabrication sequence a top electrode is deposited and patterned142 shown in FIG. 15B, and one or more vias 144 created in FIG. 15C tothe bottom electrode 138, such as by etching through the top piezo layer140. Next, as seen in FIG. 15D, a portion of the piezo layer 140, thebottom electrode 138, and the passive layer 136 are patterned, such asby lithography and etching, removing mass from a central region 146 toform the ribbed structure of the PMUT. Finally, in FIG. 15E the PMUTdiaphragm is released, such as by etching a center 148 in the backsidehandle using deep reactive-ion etching (DRIE). In FIG. 15E, the SiO₂layer 134 remains on the back side of the PMUT diaphragm. In otherembodiments, SiO₂ layer 134 may be removed.

Additional embodiments are described in this section for robustair-coupled piezoelectric micromachined ultrasonic transducers (PMUTs).The design achieves a ten fold (10×) reduction in the variation infundamental frequency created by across-wafer residual stress gradientsthat are present in the piezoelectric AlN layer.

As discussed previously, FIG. 2A and FIG. 2B illustrated a patternedPMUT structure having reduced mass/enhanced stiffness. This design wasimplemented with an example created having a thick-layered PMUTdiaphragm with an 8 μm thick passive layer and a 375 μm radius.Simultaneously, by patterning wedged shaped ribs the diaphragm mass wasreduced while maintaining the enhanced stiffness of a thick-layeredPMUT, with the wide bandwidth of the thin-layered PMUT maintained toachieve sufficient axial resolution in range measurement applications.

FIG. 16 depicts how the diaphragm mass and stiffness were independentlymodified from one another as a result of patterning the surface of thetransducer. It can be seen in the figure that ribbed designs accordingto the present disclosure (seen as the diamond shape data points) do notfollow that of flat plate designs (dashed lines through circular datum)in which mass and stiffness are dependent upon one another. Five ribbeddesigns were tested having rib widths varying from 5° to 45°.

In the present disclosure, the reduced-mass/enhanced-stiffness designimproves the robustness of the transducer to residual stress, increasestheir output pressure and maintains a wide bandwidth.

To verify the approach, patterned aluminum nitride (AlN) PMUTs werefabricated. The example transducers have a 1 μm AlN active layer, 8 μmlow-stress SiO₂ passive layer, and a 375 μm radius diaphragm. Thefractional bandwidth of the transducer is inversely proportional toquality factor Q=√{square root over (mk)}/Z₀, where m and k are the massand stiffness of the diaphragm, with Z₀ representing acoustic impedance.The bandwidth is increased by removing mass from the center of thediaphragm. The velocity frequency response is measured using LDV (LaserDoppler Vibrometry), and is compared to the frequency response simulatedby finite element method (FEM). The FEM solves a three dimensionalaxisymmetric piezo-acoustic model implemented in COMSOL.

FIG. 17A and FIG. 17B illustrate an example embodiment of a new narrowrib design (e.g., 5° wide ribs) seen in FIG. 17A, with a magnified viewseen in FIG. 17B. It should be noted that in this figure, the surface ofeach rib is partially covered by the aluminum top electrode metal, whichappears light colored in the image, while the ends of the ribs areexposed aluminum nitride which appears darker. The black central regionin the image is the exposed SiO₂ elastic layer in the center of the PMUTdiaphragm.

FIG. 18A compares simulated and measured frequency response between theoriginal flat configuration and the 5° wide rib design. FEM analysisindicates 209 kHz for the flat design, and 202 kHz for the ribbeddesign, while the measured results came in with 202 kHz for the flatdesign and 177 kHz for the ribbed design, shown with error barsindicating maximum and minimum values for these measurements. Theaverage measured center frequencies of the flat and ribbedconfigurations were found to be 202 kHz and 177 kHz, respectively.

FIG. 18B compares simulated and measured fractional bandwidth betweenthe original flat configuration and the 5° wide rib design. FEM analysisindicates fractional bandwidth of 1.7% for the flat design and 2.8% forthe 5° wide rib design. The measured fractional bandwidths were found as1.3% for the flat design and 1.5% for the 5° wide rib design, with errorbars showing maximum and minimum values for these measurements.

In addition, the FEM analysis indicates that the thick design reducesthe sensitivity of the resonance frequency to residual stress by afactor of 10 compared to thin diaphragms with the same fundamentalfrequency. The measured standard deviation of the resonance frequencyacross a 2.6 mm array is 0.3% and 0.6% for flat and ribbedconfigurations, respectively, while the across-wafer frequency variationwas 15 kHz, both a factor of 10 lower than observed in thin-diaphragmdevices fabricated on similar wafers (data not shown).

FIG. 19 compares the characteristics of a flat design (shown as openbars) with those of a ribbed design (shown as filled bars). For eachcharacteristic, the result for the ribbed design is taken as 100% andthe result for the flat design is normalized to this value. In the leftcolumn of the figure, simulated static deflection (displacement) of theflat design is seen as 63% of the simulated value obtained for the ribdesign. In the next column, the measured static deflection(displacement) for the flat design is shown to be only 51% of themeasured value obtained for the rib design. The simulated sound pressurelevel-bandwidth product (SPL.BVV) for the flat design is seen as 57% ofthe simulated value for the ribbed design. The measured maximum velocityat resonance indicate that the flat design has 61% of the maximumvelocity compared with that measured for the ribbed design. In view ofall these tests, it is seen that the ribbed design has superiorperformance in comparison to the original flat plate design.

Thus, a variable thickness diaphragm is disclosed which simultaneouslyimproves several performance metrics. Compared to the traditional flatplate design, the novel design is more robust to residual stress, haswider bandwidth, and is predicted to have higher output pressure.Therefore, this ribbed pattern design is more suitable than a flat platefor air-coupled range finding applications that require short timeconstant, high SPL output and good frequency matching of PMUTs within anarray.

From the description herein, it will be appreciated that that thepresent disclosure encompasses multiple embodiments which include, butare not limited to, the following:

1. An apparatus for ultrasonic generation and sensing, comprising: (a)at least one elastic layer; (b) at least one piezoelectric materiallayer mechanically coupled to said elastic layer; (c) at least oneelectrode of electrically conductive material disposed in electricalcontact with each side of said piezoelectric material providingelectrodes for said piezoelectric material as a backside electrode andfrontside electrode; (d) wherein a combination of said elastic layer,piezoelectric material layer, backside electrode, and frontsideelectrode form an ultrasonic diaphragm for generating ultrasonic signalsin response to applying a selected transmission voltage waveform signalbetween said backside electrode and said frontside electrode, and/or forsensing ultrasonic signals in response to sensing a received voltagewaveform signal between said backside electrode and said frontsideelectrode; and (e) wherein said diaphragm has a surface patterned withselective thinned areas through a combination of at least saidpiezoelectric material layer and said electrodes to vary the diaphragmthickness at specific locations to independently select mass andstiffness toward increasing bandwidth and reducing sensitivity toresidual stress.

2. An apparatus for ultrasonic sound generation and sensing, comprising:(a) at least one elastic layer; (b) at least one piezoelectric materiallayer mechanically coupled to said elastic layer; (c) at least oneelectrode of electrically conductive material disposed in electricalcontact with each side of said piezoelectric material providingelectrodes for said piezoelectric material as a backside electrode and afrontside electrode; (d) wherein a combination of said elastic layer,piezoelectric material layer, backside electrode, and frontsideelectrode form a diaphragm; (e) wherein said diaphragm is configured forgenerating ultrasonic signals in response to applying a selectedtransmission voltage waveform signal between said backside electrode andsaid frontside electrode, and/or for sensing ultrasonic signals inresponse to sensing a received voltage waveform signal between saidbackside electrode and said frontside electrode; and (f) wherein saiddiaphragm is patterned on its surface leaving thinned areas passingthrough at least said piezoelectric layer and said backside electrodewhich configure said diaphragm with a selected mass and stiffness towardincreasing bandwidth and reducing sensitivity to residual stress.

3. The apparatus as recited of any preceding embodiment, furthercomprising a base structure having a closed shape surrounding an openingover which said diaphragm is disposed.

4. The apparatus of any preceding embodiment, wherein said opening insaid base structure forms a backside tube.

5. The apparatus of any preceding embodiment, wherein said frontsideelectrode comprises conductive material formed in a closed shape andsurrounding an open area in which there is no conductive material.

6. The apparatus of any preceding embodiment, wherein said diaphragm ispatterned underneath the layer of said frontside electrode in an areabeneath said open area in said frontside electrode.

7. The apparatus of any preceding embodiment, wherein said diaphragm ispatterned leaving thinned areas which pass through said piezoelectriclayer and said backside electrode, as well as through a portion of saidelastic layer reducing its thickness.

8. The apparatus of any preceding embodiment, wherein said diaphragm ispatterned with multiple ribs of material from said piezoelectric layerand said backside electrode which extend in layers from beneath saidfrontside electrode into an area beneath the open region in saidfrontside electrode.

9. The apparatus of any preceding embodiment, wherein said ribs aredistributed with equal spacing extending in layers from beneath saidfrontside electrode into area beneath the open region in said frontsideelectrode.

10. The apparatus of any preceding embodiment, wherein each of said ribstapers along it length extending in the layers from beneath saidfrontside electrode into the area beneath the open region in saidfrontside electrode.

11. The apparatus of any preceding embodiment, wherein each of said ribstaper in an isosceles triangle shape or isosceles trapezoid shape.

12. The apparatus of any preceding embodiment, wherein said diaphragm ispatterned on its surface beneath the open area of said closed shape inthe frontside electrode, and also beneath a portion of said frontsideelectrode.

13. The apparatus of any preceding embodiment, wherein said patterningof said diaphragm comprises material layer remnants of said diaphragmafter said open areas in said piezoelectric layer and said electrodebackside electrode have been removed.

14. The apparatus of any preceding embodiment, wherein said diaphragm ispatterned with material added in said piezoelectric layer and saidbackside electrode to surround said open areas in said diaphragm.

15. The apparatus of any preceding embodiment, wherein said diaphragm ispatterned on its surface to decouple resonance frequency and bandwidthcharacteristics making them substantially independent variables.

16. The apparatus of any preceding embodiment, wherein saidpiezoelectric layer comprises a material selected from the group ofpiezoelectric materials consisting of Aluminum Nitride (AlN), Apatite,Barium Titanate (BaTiO₃), Berlinite (AlPO₄), various Ceramic materials,Allium Phosphate, Gallium Orthophosphate, Gallium Nitride (GaN),Lanthanum Gallium Silicate, Lead Scandium Tantalate, Lead MagnesiumNiobate (PMN), Lead Zirconate Titanate (PZT), Lithium Tantalate,Polyvinylidene Fluoride (PVDF), Potassium Sodium Tartrate, Quartz(SiO₂), Zinc Oxide (ZnO), and combinations thereof.

17. The apparatus of any preceding embodiment, wherein said elasticlayer comprises a passive material.

18. The apparatus of any preceding embodiment, wherein said passivematerial comprises Silicon (Si), Silicon Nitride (Si₃N₄), or an oxide ofSilicon, including SiO₂.

19. The apparatus of any preceding embodiment, wherein said elasticlayer comprises an active piezoelectric material.

20. The apparatus of any preceding embodiment, wherein said elasticlayer comprises a material selected from a group of piezoelectricmaterials consisting of Aluminum Nitride (AlN), Apatite, Barium Titanate(BaTiO₃), Berlinite (AlPO₄), various Ceramic materials, AlliumPhosphate, Gallium Orthophosphate, Gallium Nitride (GaN), LanthanumGallium Silicate, Lead Scandium Tantalate, Lead Magnesium Niobate (PMN),Lead Zirconate Titanate (PZT), Lithium Tantalate, PolyvinylideneFluoride (PVDF), Potassium Sodium Tartrate, Quartz (SiO₂), Zinc Oxide(ZnO), and combinations thereof.

21. The apparatus of any preceding embodiment, wherein the apparatuscomprises a piezoelectric micromachined ultrasonic transducer (PMUT).

22. The apparatus of any preceding embodiment, wherein said apparatuscomprises a piezoelectric micromachined ultrasonic transducer (PMUT)utilized in an array of PMUTs.

23. The apparatus of any preceding embodiment, wherein a surface of saiddiaphragm is patterned by selectively etching said diaphragm to removematerial from a center portion of said diaphragm.

24. The apparatus of any preceding embodiment, wherein the diaphragmsurface is selectively etched to leave radial stiffening ribs at theperimeter of the diaphragm.

25. The apparatus of any preceding embodiment, wherein the diaphragmdiameter is from approximately 100 microns to 2 millimeters and themaximum thickness of the diaphragm is from 1 micron to 40 microns.

26. The apparatus of any preceding embodiment, wherein the centerfrequency ranges from 40 kHz to 800 kHz.

27. The apparatus of any preceding embodiment, wherein an array oftransducers all having an identical nominal center frequency arefabricated on a common substrate.

28. A method of fabricating a piezoelectric micromachined ultrasonictransducer (PMUT) configured for ultrasonic generation and/or sensing,comprising: (a) forming at least one planar elastic layer from passivematerial; (b) forming a backside electrode of conductive material oversaid planar elastic layer; (b) forming at least one piezoelectricmaterial layer in having a first side in electrical content with saidbackside electrode; (c) forming a frontside electrode of conductivematerial in electrical contact with a second side of said piezoelectricmaterial layer; (d) wherein the combination of said elastic layer,piezoelectric material layer, backside electrode, and frontsideelectrode form a diaphragm of a piezoelectric micromachined ultrasonictransducer (PMUT); (e) etching through at least said piezoelectricmaterial layer and said backside electrode to form a pattern of ribs insaid diaphragm around which at least said piezoelectric material andsaid backside electrode material have been removed; and (f) whereby saidetching to form the pattern of ribs configures diaphragm mass andstiffness to increase bandwidth and reduce sensitivity to residualstress.

Although the description herein contains many details, these should notbe construed as limiting the scope of the disclosure but as merelyproviding illustrations of some of the presently preferred embodiments.Therefore, it will be appreciated that the scope of the disclosure fullyencompasses other embodiments which may become obvious to those skilledin the art.

In the claims, reference to an element in the singular is not intendedto mean “one and only one” unless explicitly so stated, but rather “oneor more.” All structural, chemical, and functional equivalents to theelements of the disclosed embodiments that are known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed as a “means plus function”element unless the element is expressly recited using the phrase “meansfor”. No claim element herein is to be construed as a “step plusfunction” element unless the element is expressly recited using thephrase “step for”.

What is claimed is:
 1. An apparatus for ultrasonic generation andsensing, comprising: (a) at least one elastic layer; (b) at least onepiezoelectric material layer mechanically coupled to said elastic layer;and (c) at least one electrode of electrically conductive materialdisposed in electrical contact with each side of said piezoelectricmaterial providing electrodes for said piezoelectric material as abackside electrode and frontside electrode; (d) wherein a combination ofsaid elastic layer, piezoelectric material layer, backside electrode,and frontside electrode form an ultrasonic diaphragm for generatingultrasonic signals in response to applying a selected transmissionvoltage waveform signal between said backside electrode and saidfrontside electrode, and/or for sensing ultrasonic signals in responseto sensing a received voltage waveform signal between said backsideelectrode and said frontside electrode; and (e) wherein said diaphragmhas a surface patterned with selective thinned areas through acombination of at least said piezoelectric material layer and saidelectrodes to vary the diaphragm thickness at specific locations toindependently select mass and stiffness toward increasing bandwidth andreducing sensitivity to residual stress.
 2. An apparatus for ultrasonicsound generation and sensing, comprising: (a) at least one elasticlayer; (b) at least one piezoelectric material layer mechanicallycoupled to said elastic layer; (c) at least one electrode ofelectrically conductive material disposed in electrical contact witheach side of said piezoelectric material providing electrodes for saidpiezoelectric material as a backside electrode and a frontsideelectrode; (d) wherein a combination of said elastic layer,piezoelectric material layer, backside electrode, and frontsideelectrode form a diaphragm; (e) wherein said diaphragm is configured forgenerating ultrasonic signals in response to applying a selectedtransmission voltage waveform signal between said backside electrode andsaid frontside electrode, and/or for sensing ultrasonic signals inresponse to sensing a received voltage waveform signal between saidbackside electrode and said frontside electrode; and (f) wherein saiddiaphragm is patterned on its surface leaving thinned areas passingthrough at least said piezoelectric layer and said backside electrodewhich configure said diaphragm with a selected mass and stiffness towardincreasing bandwidth and reducing sensitivity to residual stress.
 3. Theapparatus as recited in claim 1 or claim 2, further comprising a basestructure having a closed shape surrounding an opening over which saiddiaphragm is disposed.
 4. The apparatus as recited in claim 3, whereinsaid opening in said base structure forms a backside tube.
 5. Theapparatus as recited in claim 1 or claim 2, wherein said frontsideelectrode comprises conductive material formed in a closed shape andsurrounding an open area in which there is no conductive material. 6.The apparatus as recited in claim 5, wherein said diaphragm is patternedunderneath the layer of said frontside electrode in an area beneath saidopen area in said frontside electrode.
 7. The apparatus as recited inclaim 5, wherein said diaphragm is patterned leaving thinned areas whichpass through said piezoelectric layer and said backside electrode, aswell as through a portion of said elastic layer reducing its thickness.8. The apparatus as recited in claim 5, wherein said diaphragm ispatterned with multiple ribs of material from said piezoelectric layerand said backside electrode which extend in layers from beneath saidfrontside electrode into an area beneath the open region in saidfrontside electrode.
 9. The apparatus as recited in claim 8, whereinsaid ribs are distributed with equal spacing extending in layers frombeneath said frontside electrode into area beneath the open region insaid frontside electrode.
 10. The apparatus as recited in claim 8,wherein each of said ribs tapers along its length extending in thelayers from beneath said frontside electrode into the area beneath theopen region in said frontside electrode.
 11. The apparatus as recited inclaim 8, wherein each of said ribs taper in an isosceles triangle shapeor isosceles trapezoid shape.
 12. The apparatus as recited in claim 5,wherein said diaphragm is patterned on its surface beneath the open areaof said closed shape in the frontside electrode, and also beneath aportion of said frontside electrode.
 13. The apparatus as recited inclaim 1 or claim 2, wherein said patterning of said diaphragm comprisesmaterial layer remnants of said diaphragm after said open areas in saidpiezoelectric layer and said electrode backside electrode have beenremoved.
 14. The apparatus as recited in claim 1 or claim 2, whereinsaid diaphragm is patterned with material added in said piezoelectriclayer and said backside electrode to surround said open areas in saiddiaphragm.
 15. The apparatus as recited in claim 1 or claim 2, whereinsaid diaphragm is patterned on its surface to decouple resonancefrequency and bandwidth characteristics making them substantiallyindependent variables.
 16. The apparatus as recited in claim 1 or claim2, wherein said piezoelectric layer comprises a material selected fromthe group of piezoelectric materials consisting of Aluminum Nitride(AlN), Apatite, Barium Titanate (BaTiO₃), Berlinite (AlPO₄), variousCeramic materials, Allium Phosphate, Gallium Orthophosphate, GalliumNitride (GaN), Lanthanum Gallium Silicate, Lead Scandium Tantalate, LeadMagnesium Niobate (PMN), Lead Zirconate Titanate (PZT), LithiumTantalate, Polyvinylidene Fluoride (PVDF), Potassium Sodium Tartrate,Quartz (SiO₂), Zinc Oxide (ZnO), and combinations thereof.
 17. Theapparatus as recited in claim 1 or claim 2, wherein said elastic layercomprises a passive material.
 18. The apparatus as recited in claim 17,wherein said passive material comprises Silicon (Si), Silicon Nitride(Si₃N₄), or an oxide of Silicon, including SiO₂.
 19. The apparatus asrecited in claim 1 or claim 2, wherein said elastic layer comprises anactive piezoelectric material.
 20. The apparatus as recited in claim 19,wherein said elastic layer comprises a material selected from a group ofpiezoelectric materials consisting of Aluminum Nitride (AlN), Apatite,Barium Titanate (BaTiO₃), Berlinite (AlPO₄), various Ceramic materials,Allium Phosphate, Gallium Orthophosphate, Gallium Nitride (GaN),Lanthanum Gallium Silicate, Lead Scandium Tantalate, Lead MagnesiumNiobate (PMN), Lead Zirconate Titanate (PZT), Lithium Tantalate,Polyvinylidene Fluoride (PVDF), Potassium Sodium Tartrate, Quartz(SiO₂), Zinc Oxide (ZnO), and combinations thereof.
 21. The apparatus asrecited in claim 1 or claim 2, wherein the apparatus comprises apiezoelectric micromachined ultrasonic transducer (PMUT).
 22. Theapparatus as recited in claim 1 or claim 2, wherein said apparatuscomprises a piezoelectric micromachined ultrasonic transducer (PMUT)utilized in an array of PMUTs.
 23. The apparatus as recited in claim 1or claim 2, wherein a surface of said diaphragm is patterned byselectively etching said diaphragm to remove material from a centerportion of said diaphragm.
 24. The apparatus as recited in claim 1 orclaim 2, wherein the diaphragm surface is selectively etched to leaveradial stiffening ribs at the perimeter of the diaphragm.
 25. Theapparatus as recited in claim 1 or claim 2, wherein the diaphragmdiameter is from approximately 100 microns to 2 millimeters and themaximum thickness of the diaphragm is from 1 micron to 40 microns. 26.The apparatus as recited in claim 1 or claim 2, wherein the centerfrequency ranges from 40 kHz to 800 kHz.
 27. The apparatus as recited inclaim 1 or claim 2, wherein an array of transducers all having anidentical nominal center frequency are fabricated on a common substrate.28. A method of fabricating a piezoelectric micromachined ultrasonictransducer (PMUT) configured for ultrasonic generation and/or sensing,comprising: (a) forming at least one planar elastic layer from passivematerial; (b) forming a backside electrode of conductive material oversaid planar elastic layer; (c) forming at least one piezoelectricmaterial layer in having a first side in electrical content with saidbackside electrode; (d) forming a frontside electrode of conductivematerial in electrical contact with a second side of said piezoelectricmaterial layer; (e) wherein the combination of said elastic layer,piezoelectric material layer, backside electrode, and frontsideelectrode form a diaphragm of a piezoelectric micromachined ultrasonictransducer (PMUT); (f) etching through at least said piezoelectricmaterial layer and said backside electrode to form a pattern of ribs insaid diaphragm around which at least said piezoelectric material andsaid backside electrode material have been removed; and (g) whereby saidetching to form the pattern of ribs configures diaphragm mass andstiffness to increase bandwidth and reduce sensitivity to residualstress.